Everything is plugged in the way it should be, my preferences are all correct, my monitors are on, and none of the tracks are muted or off, etc. If I try playing MIDI the levels show me that there is sound. I have tried restarting both Ableton AND my computer, and this project still won't play any sound when it should. Does anyone have any ideas that aren't quite so obvious?

Audio driver selected correctly in Lives preferences? Speakers plugged into correct output? Volume control is not at zero? No mutes or bypasses selected on your interface? Other programs make sound? The project you opened actually has sound (master meter is showing a level)?